11 Using Indexes in Database Applications

Indexes are optional structures, associated with tables and clusters, which allow SQL queries to execute more quickly. Just as the index in this guide helps you locate information faster than if there were no index, an Oracle Database index provides a faster access path to table data. You can use indexes without rewriting any queries. Your results are the same, but you see them more quickly.

When to Use Domain Indexes

A domain index (also called an application domain index) is a customized index specific to an application that was implemented using a data cartridge (for example, a search engine or geographic information system).

When to Use Function-Based Indexes

A function-based index computes the value of an expression that involves one or more columns and stores it in the index. The index expression can be an arithmetic expression or an expression that contains a SQL function, PL/SQL function, package function, or C callout. Function-based indexes also support linguistic sorts based on collation keys, efficient linguistic collation of SQL statements, and case-insensitive sorts.

A function-based index improves the performance of queries that use the index expression (especially if the expression computation is intensive). However:

The database must also evaluate the index expression to process statements that do not use it.

Function-based indexes on columns that are frequently modified are expensive for the database to maintain.

The optimizer can use function-based indexes only for cost-based optimization, while it can use indexes on columns for both cost-based and rule-based optimization.

Note:

A function-based index cannot contain the value NULL. Therefore, either ensure that no column involved in the index expression can contain NULL or use the NVL function in the index expression to substitute another value for NULL.

Oracle Database treats descending indexes as if they were function-based indexes.

Advantages of Function-Based Indexes

A function-based index has these advantages:

A function-based index increases the number of situations where the database can perform an index range scan instead of a full index scan.

An index range scan typically has a fast response time when the WHERE clause selects fewer than 15% of the rows of a large table. The optimizer can more accurately estimate how many rows an expression selects if the expression is materialized in a function-based index.

Oracle Database represents the index expression as a virtual column, on which the ANALYZE statement can build a histogram.

A function-based index precomputes and stores the value of an expression.

Queries can get the value of the expression from the index instead of computing it. The more queries that need the value and the more intensive computation the index expression gets, the index improves application performance.

You can create a function-based index on an object column or REF column.

The index expression can be the invocation of a method that returns an object type.

A function-based index lets you perform more powerful sorts.

The index expression can invoke the SQL functions UPPER and LOWER for case-insensitive sorts (as in Example 11-3) and the SQL function NLSSORT for linguistic-based sorts.

Disadvantages of Function-Based Indexes

The optimizer can use a function-based index only for cost-based optimization, not for rule-based optimization.

The cost-based optimizer uses statistics stored in the dictionary. To gather statistics for a function-based index, invoke either DBMS_STATS.GATHER_TABLE_STATS or DBMS_STATS.GATHER_SCHEMA_STATS .

The database does not use a function-based index until you analyze the index itself and the table on which it is defined.

To analyze the index and the table on which the index is defined, invoke either DBMS_STATS.GATHER_TABLE_STATS or DBMS_STATS.GATHER_SCHEMA_STATS.

The database does not use function-based indexes when doing OR expansion.

You must ensure that any schema-level or package-level PL/SQL function that the index expression invokes is deterministic (that is, that the function always return the same result for the same input).

You must declare the function as DETERMINISTIC, but because Oracle Database does not check this assertion, you must ensure that the function really is deterministic.

If you change the semantics of a DETERMINISTIC function and recompile it, then you must manually rebuild any dependent function-based indexes and materialized views. Otherwise, they report results for the prior version of the function.

If the index expression is a function invocation, then the function return type cannot be constrained.

Because you cannot constrain the function return type with NOTNULL, you must ensure that the query that uses the index cannot fetch NULL values. Otherwise, the database performs a full table scan.

The index expression cannot invoke an aggregate function.

A bitmapped function-based index cannot be a descending index.

The data type of the index expression cannot be VARCHAR2, RAW, LONGRAW, or a PL/SQL data type of unknown length.

That is, you cannot index an expression of unknown length. However, you can index a known-length substring of that expression. For example:

Example: Function-Based Index for Precomputing Arithmetic Expression

You can create composite indexes to computer arithmetic expressions.

Example 11-1 creates a table with columns a, b, and c; creates an index on the table, and then queries the table. The index is a composite index on three columns: a virtual column that represents the expression a+b*(c-1), column a, and column b. The query uses the indexed expression in its WHERE clause; therefore, it can use an index range scan instead of a full index scan.

Example: Function-Based Indexes on Object Column

In Example 11-2, assume that the object type Reg_obj has been defined, and that it stores information about a city. The example creates a table whose first column has type Reg_obj, a deterministic function with a parameter of type Reg_obj, and two function-based indexes that invoke the function. The first query uses the first index to quickly find cities further than 1000 miles from the equator. The second query uses the second index (which is composite) to quickly find cities where the temperature delta is less than 20 and the maximum temperature is greater than 75. (The table is not populated for the example, so the queries return no rows.)

Example: Function-Based Index for Language-Dependent Sorting

You can use the NLSSORT API for language-dependent sorting.

Example 11-4 creates a table with one column, NAME, and a function-based index to sort that column using the collation sequence GERMAN, and then selects all columns of the table, ordering them by NAME. Because the query can use the index, the query is faster. (Assume that the query is run in a German session, where NLS_SORT is GERMAN and NLS_COMP is ANSI. Otherwise, the query would have to specify the values of these Globalization Support parameters.)